This invention relates to revolving rolls in general, and, more particularly, to the measurement of the temperature of the peripheral surface of a revolving roll.
The exact measurement of the surface temperature of revolving rolls is still today a difficult technical problem. Especially for rolls which are used in the treatment of paper, plastic films, textile and nonwoven materials, it is sometimes vital to maintain temperatures with a deviation of less than 1 C. This deviation limit must be maintained both as regard to the absolute value and as to deviation from a mean value determined along a roll several meters long. An especially striking example is the compaction of nonwoven material made from thermoplastic synthetic fibers. Here the temperature must be such that during passage of the laid web through the roll nip the fibers are welded together just sufficiently for the web to hold together, but that, at the same time, the bond is limited to a certain degree so that the finished web remains rigid but does not become too boardy. For this, a very exact sensing and control of the temperature in dependence on the respective operating speed is essential.
The nexus of problems of exact temperature measurement on such rolls is connected with the fact, among others, that temperature sensors cannot be accommodated in the roll periphery. This would require bores in the peripheral surface resulting in an inhomogeneity of the peripheral surface. Such an inhomogeneity would be reflected in the roll surface and could lead to harmful stress concentrations and perturbations of the temperature distribution. Also, transmission of the measured values from the revolving roll to the outside would be costly.
In practice, therefore, only measurement at the roll periphery from the outside enters into consideration. For this purpus methods with contacting temperature sensors and contactless methods operating with radiation thermometers exist. The methods involving abutting contact of the temperature sensors are indeed very exact, because they can utilize heat conduction for the transmission to the sensor. They have the disadvantage, however, that in many cases they cannot be employed at any rate for a protracted temperature observation. Often, in fact, the rolls are precision machined on the surface according to the desired product quality, e.g., ground and polished. Such a surface would, due to the friction of a contacting fixed temperature sensor, show grinding traces after a short service period. These traces would then appear in the product, e.g., the paper web or plastic surface. Hence, this method is not acceptable. Contactless arrangements, i.e., those based on a radiation measurement, do not have this disadvantage, but they depend on the roll surface having a constant emission factor. But this, precisely, is often not the case in the operating zone of a roll, because adhering moisture and also tarnishing of the roll surface at higher operating temperatures cause variations of the emission factor. As a result, the measured radiation does not give a clear indication of the temperatures actually prevailing at the roll periphery.
Thus, there is a need to be able to reliably make an exact measurement of the temperature of the periphery of revolving rolls over a protracted service period.